This invention relates to a surface analysis system.
In general, a surface analysis system is an instrument which provides a quantitative assessment of the properties of a surface, for example, of a semiconductor wafer.
One technique for assessing the quality of the surface of a semiconductor wafer is by performing a contact angle measurement. A contact angle measurement is a simplified method of characterizing the interfacial tension present between a solid, a liquid, and a vapor. The contact angle is defined as the angle between the support surface and the tangent to the profile of the droplet at the point of contact of the liquid droplet with the surface.
The value of the contact angle of the liquid droplet will depend upon surface wettability. If perfect or complete wetting takes place between the liquid and the surface, the solid surface energy exceeds the liquid surface tension, the droplet spreads out over the surface, and the contact angle approaches zero degree. Thus, the droplet has a flatter, lower profile shape, for example, in the case when wine wets or "sheets" the surface of a glass. On the other hand, when a droplet of a high surface tension liquid rests on a solid of low energy, wetting is only partial, and the resulting contact angle will lie in the range of 0 to 180 degrees. In this case, the liquid surface tension will cause the droplet to form a spherical shape (the lowest energy shape); one example is the way water droplets "bead up" on the surface of a freshly waxed car. A detailed description of contact angle measurements is found in "Contact Angle, Wettability and Adhesion", edited by K. L. Mittal, VSP BV, The Netherlands, 1993, which is incorporated herein by reference.
Contact angle measurements can be used to determine surface cleanliness, primer efficacy, coating uniformity, and oxide thickness. One application in which measuring the contact angle is particularly useful is in evaluating the quality of hexamethyldisilazane (HMDS) deposited on a semiconductor wafer surface. HMDS is typically deposited over portions of a wafer prior to depositing a layer of photoresist which serves as a mask in a subsequent etching process and is then removed. Depositing HMDS is recognized as a critical surface preparation step to ensure that the photoresist adheres properly to substrate so that "under etching" is minimized.
Devices are known for determining the contact angle of a droplet, both by direct measurement of the angle and by indirect calculation based upon measurement of the height, width, or radius of the droplet. One common technique involves projecting a silhouette image of the deposited droplet on a projection screen and determining the contact angle by direct or indirect measurements taken from the silhouette.